1. Field of the Invention
This invention relates to catalytic converters for the exhaust systems of internal combustion engines; more particularly it relates to a catalytic converter (hereinafter referred to as a "coupled type catalytic converter") in which at least two catalyst units are arrayed in series in the direction of exhaust gas flow, with a suitable gap between the units in which is mounted a secondary air supply pipe.
2. Prior Art
A coupled type catalytic converter comprises one catalyst unit (hereinafter referred to as the first catalyst unit) disposed at the exhaust gas inlet end (upstream end) of the converter container for effecting chiefly the reduction reaction, and a separate catalyst unit (hereinafter referred to as the second catalyst unit) disposed at the exhaust gas outlet end (downstream end) of the converter container for effecting chiefly the oxidation reaction, so that all harmful components in the exhaust gas, irrespective of their nature, can be converted into cleaned or purified gases to prevent environmental pollution. Since a large amount of the oxygen in the exhaust gas is consumed in the first catalyst unit before the exhaust gas flows into the second catalyst unit, there is not enough oxygen left for the second catalyst unit to fully perform the oxidizing function. Therefore, it is accepted practice to supply secondary air from a secondary air supply pipe to the upstream portion of the second catalyst unit, to provide sufficient oxygen gas such that the second catalyst unit can fully effect the oxidation reaction.
Generally, catalytic converters such as coupled type converters are placed under very severe thermal conditions, since they carry high temperature exhaust gases. In coupled type converters, therefore, the difference in thermal expansion between the container and the secondary air supply pipe causes thermal stress and thermal fatigue, while the engine vibration causes mechanical fatigue, so that the air supply pipe may be prematurely damaged or deformed, thereby interrupting the supply of secondary air and degrading the purification performance of the coupled type catalytic converter.